Evaluation of urine excretion from dairy cows under two farm systems using urine sensors

Urine nitrogen (N) is the main source of N leaching from pasture grazed with dairy cows. The aim of this work was to compare urine N production by groups of lactating dairy cows in two contrasting systems (Future (F) and Current (C)). The systems differed in: annual N fertiliser inputs to the pasture (50 vs. 150 kg N ha-1 for F and C); stocking rate (2.6 (F) vs. 3.2 (C) cows ha-1); and housing cows for 6 hours per day in autumn (F). Continuous measurements of urine volume, time and N concentration were made over three days in both early summer and in autumn using urine sensors on 9 (F) or 11 (C) cows. There was no difference between the two systems in average urine N concentration (5.5. g N L-1) or urine volume (1.98 L), averaged over the two sampling periods. There was a tendency for increased urination frequency in F cows (c. 14.8 vs. 13.5 urinations day-1, P=0.1), attributed to greater feed intake per cow in the F treatment. Estimated daily urine N production was c. 190 g N cow-1, with no difference between the two systems. Although there was no difference in urinary N production per cow, there was less urine N per ha deposited in the lower N input F system (504 vs. 585 g N ha-1, P<0.05), a 14% reduction. We conclude this was due to less pasture grown and eaten (and less N eaten) per ha in the lower N input F system, rather than due to per cow differences. It was estimated that removing cows from the paddock for 6 hours per day in autumn would reduce daily urine deposition on to the paddock by c. 25%. We would expect less N leaching as a result of less urine N deposition in the F system arising from a combination of lower N inputs and removing cows for periods of the day. Urine sensors are a useful tool for studying the grazing system.